Digitally controlled color ink jet printing capability is accomplished by one of two technologies referred to as “drop-on-demand” and “continuous stream,” respectively. Both require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a medium. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce, in general, up to several million perceived color combinations.
Drop-on-demand ink jet printing, provides ink droplets for impact upon a print medium using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print medium and strikes the print medium. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle thus helping to keep the nozzle clean. Conventional drop-on-demand ink jet printers utilize a pressurization actuator to produce the ink jet droplet at orifices of a print head. Typically, one of two types of actuators are used including heat actuators and piezoelectric actuators. With heat actuators, a heater, placed at a convenient location, heats the ink. This causes a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a mechanical stress in the material, thereby causing an ink droplet to be expelled. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.
By contrast, continuous stream ink jet printing, uses a pressurized ink source which produces a continuous stream of ink droplets. Electrostatic charging devices are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink droplets are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or discarded. When printing is desired, the ink droplets are not deflected and allowed to strike a print medium. Alternatively, deflected ink droplets may be allowed to strike the print medium, while non-deflected ink droplets are collected in the ink capturing mechanism. Continuous ink jet printing devices are faster than drop on demand devices and produce higher quality printed images and graphics. However, each color printed requires an individual droplet formation, deflection, and capturing system.
One of the problems associated with both types of ink jet technologies is that of printhead reliability. For continuous ink jet printers a common problem is initial stream instability that occurs when the printheads are turned on during start-up. Initial stream instability is often due to dynamics associated with surface wetting near the nozzles as well as any differential wetting that results from surface contamination. Initial aberrations of the ink stream may also originate from the presence of air bubbles in the printhead. Low ink pressures during the start-up and shut-down transitions is another common source of stream instability in the form of temporary jet misdirection. Prior art methods of coping with such instabilities require the use of a cap or nozzle that move over the printhead nozzles at shut-down and/or start-up time and effectively contain the ink streams and/or ink droplets emanating from the print head at start-up and/or shutdown time.
In addition to stream instabilities that occur during start-up and shut-down, ink jet printheads develop problems from ink which has dried around nozzles after a period of operation. A combination of dried ink, paper fibers and dust can result in partial or complete blocking of nozzle apertures. Periodic maintenance is normally performed to remove dried ink and these other contaminates from the nozzle plate and ink collecting structures. It is well known in the art to rinse the head with water and blow air across it to perform the maintenance operation. An exemplary technique for cleaning with fluids (including air) is given in U.S. Pat. No. 4,970,535 to Oswald et al. in 1990. This method includes enclosing the print head with a cavity having an inlet and an outlet such that a fluid is directed through the inlet and cavity at an angle that is substantially tangential to the nozzle aperture. Ink disposed around the nozzles is thusly carried away through the outlet. Other prior art techniques require the use of a wiping device for dried ink from the nozzles. For instance physical wipers, such as squeegees and cloth wipes are moved across or blotted against the face.
A final printhead reliability problem is caused by the storage of printheads between periods of use wherein ink dries out in and adjacent to the nozzles. One solution is to keep a moist or solvent rich environment proximate to the nozzles during storage. For example, U.S. Pat. No. 4,626,869 to Piatt in 1985 describes a system wherein the critical components of the printhead assembly are stored in a wet condition.
To provide for the maintenance operations necessary to prevent the aforementioned reliability problems, the printer may include a built-in start-up station, also called a home station, which is located at the side of the printhead. The printhead is moved over and into sealed relation with a chamber of the home station where various cleaning, drying and diagnostic operations are performed. While the procedures performed by such start-up stations are quite effective, the addition of such stations add considerable complexity and cost to the printing apparatus.
Clearly, there is a need for a mechanism that effectively provides the needed maintenance and cleaning operations on the printhead of an ink jet printer without the need for a dedicated start-up maintenance station. Ideally, such operations could be implemented by structures easily integrated into the printhead itself to simplify the printer structure and reduce printer fabrication costs. Finally, it would be desirable if at least some of the maintenance operations could be implemented or facilitated by preexisting structures within the printer that are normally used for other purposes to further lower printer construction costs.